1. Field of the invention
The present invention relates to a method for selecting the optimum number of phases for a converter, and more particularly to a method for selecting the optimum number of phases for a converter and a system using the same, which selects a duty range using an input voltage and an output voltage, obtains ripple values for multiple phases in the duty range, and selects the optimum number of phases using the corresponding ripple values.
2. Description of the Prior Art
Recently, there has been a growing interest in environmentally friendly power generation due to the increase of power demand with an abrupt development of industry, environmental pollution problems caused by global warming, and the depletion of fossil fuels. Representative environmentally friendly power generation types are fuel cells, photovoltaics, and wind turbines. Particularly, since fuel cells do not have limited power generation resources, like sunlight and wind force, they are environmentally friendly, and make little noise in power generation, and they are viewed as a model technique for environmentally friendly power generation. Part of converting an output voltage of such fuel cells involves a DC-to-DC (DC/DC) converter and a DC-to-AC (DC/AC) inverter, and particularly to a boost converter that boosts a low voltage of the fuel cells up to a DC voltage of a specified level. Such a converter may be a single-phase converter or a multi-phase converter which is composed of multiple inductors and switch elements and uses one capacitor.
FIG. 1 is a view schematically illustrating the configuration of a conventional single-phase boost converter.
Referring to FIG. 1, the conventional single-phase boost converter includes an inductor 10 storing power of an input voltage, a capacitor 40 receiving and boosting the power supplied from the inductor 10 and outputting the boosted power to load directly or through an inverter, and a switch element 20 being turned on to store the supplied power of the input voltage in the inductor 10 and being turned off to make the capacitor 40 boost the power supplied from the inductor 10 and output the boosted power to the load directly or through the inverter.
The conventional single-phase boost converter also includes a diode element 30 preventing the current of an output terminal from flowing backward to an input terminal.
With reference to FIG. 2, the operation of the conventional single-phase boost converter will be described.
At the first time t1 when the switch element 20 is turned on, the power of the input voltage is stored in the inductor 10. Accordingly, a switch current IS, which gradually ascends as the power is stored in the inductor 10, flows through the switch element 20. Then, at the second time t2 when the switch element 20 is turned off, the power supplied from the inductor 10 is boosted by the capacitor 40, and the boosted power is supplied to the load directly or through the inverter.
Accordingly, a diode current ID, which gradually descends as the power stored in the inductor 10 is supplied to the capacitor 40, flows through the diode element 30. Through the above-described operation, an inductor current IL, which is the sum of the ascending switch current IS and the descending diode current ID, flows through the inductor 10. Accordingly, an inductor current ripple ΔIL is generated through the inductor by the ascending switch current IS and the descending diode current ID.
This ripple exerts a bad influence upon the system.
On the other hand, in addition to the single-phase converter as illustrated in FIG. 1, there is a multi-phase converter that is a parallel type boost converter configured to have multiple phases which have the same duty and a specified phase difference.
According to such a multi-phase converter, since input current ripples of respective phases have a specified phase difference even at a low switching frequency, the size of the whole input current ripples is decreased, and this causes the capacity and the size of the inductor to be reduced. Also, output voltage ripples are abruptly decreased due to the phase difference between the phases, and this causes the capacity and the size of the capacitor also to be abruptly reduced.
FIG. 3 schematically illustrates a three-phase converter as an example of a multi-phase converter.
As illustrated in FIG. 3, the three-phase inverter is provided with three inductors and three switch elements, and uses one capacitor.
As illustrated in FIG. 3, the general three-phase converter includes first to third inductors 112, 114, and 116 storing power of an input voltage, a capacitor 140 receiving and boosting the power from the first to third inductors 112, 114, and 116, and outputting the boosted power to load directly or through an inverter, a first switch element 122 being turned on to store the supplied power of the input voltage in the first inductor 112 and being turned off to make the capacitor 140 boost the power supplied from the first inductor 112 and output the boosted power to the load directly or through the inverter, a second switch element 124 being turned on to store power supplied from fuel cells in the second inductor 114 and being turned off to make the capacitor 140 boost the power supplied from the second inductor 114 and output the boosted power to the load directly or through the inverter, and a third switch element 126 being turned on to store the power supplied from the fuel cells in the third inductor 116 and being turned off to make the capacitor 140 boost the power supplied from the third inductor 116 and output the boosted power to the load directly or through the inverter.
The general three-phase converter also includes a first diode element 132 connected between the first inductor 112 preventing the current of an output terminal from flowing backward to an input terminal and the capacitor 140, a second diode element 134 connected between the second inductor 114 and the capacitor 140, and a third diode element 136 connected between the third inductor 116 and the capacitor 140.
The three-phase converter as configured above performs a switching operation with a phase delay of 120° (360°/the number of phases) and at the same switching frequency. Due to the above described configuration and switching method, the three-phase converter has the advantages of reducing the input current ripples, element current stress, element size and current rate, output voltage ripples, capacitor voltage stress, and the like. During the operation of the three-phase converter, periods where three-phase switching overlaps or does not overlap occur according to the duty thereof.
In a period where the three-phase switching overlaps, the size of the input current ripples linearly increases in the same manner as a general converter, while in other periods, the size of the input current ripples and the size of the output voltage ripples decrease at a specified rate, and the frequency increases to twice the number of phases.
Accordingly, in manufacturing converters, many developers frequently utilize multi-phase converters. However, there has been no special basis in determining the number of phases in a multi-phase converter. Accordingly, developers have properly selected the number of phases, e.g. three phases, four phases, or the like, according to their experiences or circumstances, and have manufactured converters having the selected number of phases.